Search results for "force"
showing 10 items of 3423 documents
Synthesis of chiral oxazolidin-2-ones from N-alkoxycarbonyl amino epoxides: a computational studyElectronic supplementary information (ESI) available…
2002
threo-N-Alkoxycarbonylamino epoxides 5a–d, containing the oxazolidine moiety, were converted into trans-4,5-disubstituted-2-oxazolidin-2-ones 2 with total regio- and stereoselection by means of nucleophilic intramolecular attack of the carbamate moiety to the protonated oxirane ring. Theoretical calculations confirmed both the regioselection and the preference of the cyclocarbamation reaction vs. the intermolecular attack by the solvent, arising from different behaviour in comparison with the analogous iodonium ions.
Solvent Selection in Liquid Chromatography
2013
Abstract Many solvents can be used to prepare mobile phases in liquid chromatography. Also, mixtures of solvents at different ratios are used to modify the mobile phase properties. This can make solvent selection for method development a hard task, unless suitable guidelines are followed. This chapter summarizes the most common strategies used by skilled chromatographists in reversed-phase, normal-phase, and hydrophilic interaction liquid chromatography. These are based on considerations about the global polarity of solutes, stationary phase and mobile phase, which determine the elution strength, and on the particular profile of the contributions of intermolecular interactions to the global…
Interaction of explicit solvent with hydrophobic/philic/charged residues of a protein: Residue character vs. conformational context
1998
Molecular dynamics simulations of model solutes in explicit molecular water have recently elicited novel aspects of the strong nonpair additivity of the potential of mean force (PMF) and related solvent-induced forces (SIFs) and hydration. Here we present the results of the same type of work on SIFs acting on bovine pancreatic trypsin inhibitor (BPTI) at single residue/sidechain resolution. In this system, nonpair additivity and the consequent dependence of SIFs on the protein conformational context are sufficiently strong to overturn SIFs on some individual residues, relative to expectations based on their individual characters. This finding calls for a revisitation and offers a richer and…
Intramolecular proton transfer of serine in aqueous solution. Mechanism and energetics
2000
Serine amino acid in aqueous solution is theoretically studied at the B3PW91/6-31+G** level using a dielectric continuum solvent model. Neutral and zwitterionic structures in the gas phase and in solution are described and the proton-transfer mechanism is discussed. A neutral conformation in which the carboxyl hydrogen atom is already oriented toward the amino group seems to be the absolute energy minimum in the gas phase and the most stable neutral form in solution. The absolute energy minimum in solution is a zwitterionic form. The energy barrier for proton transfer is predicted to be very small, in particular when zero-point-energy contributions are added. Our calculations allow the dyna…
Stepwise synthesis and selective dimerisation of bis- and trisloop tetra-urea calix[4]arenes.
2008
Tetra-urea calix[4]arenes substituted with four mono- or bisalkenyl residues have been converted into bis- or tetraloop compounds by intramolecular olefin metathesis, with use of a tetratosylurea calix[4]arene as template. The same strategy has now been used to synthesise trisloop compounds and bisloop compounds with adjacent loops, completing the series of the loop-containing tetra-urea derivatives. A tetra-urea calix[4]arene of the AABB type, where A stands for a bisalkenyl- and B for a monoalkenyl-substituted urea unit, was used as precursor for the three loops. It was easily synthesised from a tetraamino calix[4]arene in which two adjacent amino groups were Boc-protected. The ABCB-type …
Gas phase infrared multiple photon dissociation spectra of positively charged sodium bis(2-ethylhexyl)sulfosuccinate reverse micelle-like aggregates
2011
The capability of infrared multiple photon dissociation (IRMPD) spectroscopy to gain structural information on surfactant-based supramolecular aggregates has been exploited to elucidate intermolecular interactions and local organization of positively charged sodium bis(2-ethylhexyl)sulfosuccinate (AOTNa) aggregates in the gas phase. A detailed analysis of the stretching modes of the AOTNa CO and SO(3)(-) head groups allows one to directly probe their interactions with sodium counterions and to gain insight in their organization within the aggregate. Similarities and differences of the IRMPD spectra as compared to the infrared absorption spectrum of micellized AOTNa in CCl(4) have been analy…
Isolated glyoxylic acid-water 1:1 complexes in low temperature argon matrices
2015
Abstract The 1:1 hydrogen bonded complexes between glyoxylic acid (GA) and water are studied in low temperature argon matrices. Four different complex structures were found in deposited matrices. The lowest energy conformer (T1) of GA was found to form complex, where the water molecule was attached to the opposite side of the intramolecular hydrogen bond in the molecule (T1B). Interestingly, this complex was estimated to be +8.0 kJ mol −1 higher in energy than the most stable structure (T1A), where the water is inserted into the internal hydrogen bond, and also found in solid argon but in smaller abundance. For the second-lowest energy conformer of GA (T2), the two lowest-energy complex str…
Magnetic Langmuir-Blodgett films of ferritin with different iron contents.
2006
Magnetic Langmuir-Blodgett films of four ferritin derivatives with different iron contents containing 4220, 3062, 2200, and 1200 iron atoms, respectively, have been prepared by using the adsorption properties of a 6/1 mixed monolayer of methyl stearate (SME) and dioctadecyldimethylammonium bromide (DODA). The molecular organization of the mixed SME/DODA monolayer is strongly affected by the presence of the water-soluble protein in the subphase as shown by pi-A isotherms, BAM images, and imaging ellipsometry at the water-air interface. BAM images reveal the heterogeneity of this mixed monolayer at the air-water interface. We propose that the ferritin is located under the mixed matrix in thos…
Experimental assessment of application of gravimetric measurements for pneumatic conveying in horizontal pipelines
2008
Abstract Available theoretical considerations and practical experiments usually concern aspiration in the vertical pipelines, where the procedure of making measurements is universally known and it does not express any reservations. However, it is often necessary to conduct test in the systems of horizontal conveying of solgas, where the conditions of taking a representative sample, even for the sake of the significant effect of the gravitational force, many vary from those suggested for measurements in the systems with vertical flow. The present paper contains the results of tests which determine usability of the gravimetric method in measurements of flow parameters of two-phase gas–solid m…
Optimization of Reactive Force Field Simulation: Refactor, Parallelization, and Vectorization for Interactions
2022
Molecular dynamics (MD) simulations are playing an increasingly important role in many areas ranging from chemical materials to biological molecules. With the continuing development of MD models, the potentials are getting larger and more complex. In this article, we focus on the reactive force field (ReaxFF) potential from LAMMPS to optimize the computation of interactions. We present our efforts on refactoring for neighbor list building, bond order computation, as well as valence angles and torsion angles computation. After redesigning these kernels, we develop a vectorized implementation for non-bonded interactions, which is nearly $100 \times$ 100 × faster than the management processing…